A projecting rim or edge on the opposite side of something, used to strengthen, seal, or position parts relative to each other.
From counter- + flange (Dutch flange, a raised edge). This term emerged in mechanical engineering in the 19th century.
Pipe fitters know that counterflange seals work by creating pressure from opposite sides—it's like pinching something between two fingers instead of holding it with one, and it holds much better.
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